Control of flashing in fireflies has proved to be a particularly interesting and valuable neuromotor system for study for two reasons. First, both a major experimental probe (applied external light signals) and the motor response (light emission) are readily controllable and measurable non-invasively. Second, certain fireflies, along with man, are unique in the animal kingdom in ability to achieve synchronization of rhythmic activity within groups of individuals. Synchronized flashing of fireflies is thus a model for control aspects of human marching, dancing and orchestral music-making. Our work over several years on certain Oriental fireflies showed that individuals 'get into step' via a resetting of the timing cycle of the endogenous pacemaker in the brain. This resetting enables an individual to duplicate foreign rhythms different from his normal frequency and to entrain both to slower and faster tempos. Our recent work has focused on a local firefly, Photinus pyralis, in which flash synchronization occurs on a smaller scale than in the Orient, and by a different mechanism involving short-latency sequential photic flash-triggering. Considered as a resetting of the timing cycle, entrainment in the American species involves an immediate phase-advance of the pacemaker to the end of its excitation cycle, where the Oriental resetting involves phase retardation back to the start of the cycle. This work used video photography, a technique which opens revolutionary possibilities for bioluminescence study. The physiological work on activity synchronization has led also to the companion question of normal biological function. Since light-emission in fireflies is used primarily for courtship signaling, much interest centers on how such a cooperative group activity can promote reproduction of individuals, as it must, according to evolutionary theory, to be genetically fixed by natural selection. Accordingly I have put considerable effort into models in which flash synchronization might confer sexual benefit, and the observed behavioral sequence has been interpreted as a cooperative interaction involving long-term net gain from short-term sacrifice.